Micromachined gyroscopes with 2-DOF sense modes allowing interchangeable robust and precision operation

ABSTRACT

A z-axis gyroscope design is presented with a 2-degree of freedom (DOF) sense mode allowing interchangeable operation in either precision (mode-matched) or robust (wide-bandwidth) modes. This is accomplished using a complete 2-DOF coupled system which allows for the specification of the sense mode resonant frequencies and coupling independent of frequency. By decoupling the frame connecting the sense system to a central anchor, x-y symmetry is preserved while enabling a fully coupled 2-DOF sense mode providing control over both the bandwidth and the amount of coupling independent of operational frequency. The robust mode corresponds to operation between the 2-DOF sense mode resonant frequencies providing a response gain and bandwidth controlled by frequency spacing. Precision mode of operation, however, relies on mode-matching the drive to the second, anti-phase sense mode resonant frequency which can be designed to provide a gain advantage over a similar 1-DOF system.

RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 12/605,178, filed on Oct. 23, 2009, which is incorporated herein byreference and to which priority is claimed pursuant to 35 USC 120.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with Government support under Grant No. 0409923,awarded by the National Science Foundation. The Government has certainrights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the field of micromachined gyroscopes, inparticular gyroscope designs with 2-DOF (two degrees of freedom) sensemodes allowing interchangeable operation in either precision or robustmodes.

2. Description of the Prior Art

Micromachined vibratory gyroscopes operate based on the Coriolis effectwhere a rotation induced energy transfer occurs between two orthogonalvibrational modes, commonly referred to as drive and sense.Conventionally, these modes are realized as single degree of freedom(DOF) dynamic systems with their own associated resonant frequenciesgiving rise to two differing methods of operation: mode-matched ormismatched. In mode-matched devices, the drive and sense resonantfrequencies are equal allowing the output of the sensor to be increasedproportional to the sense mode quality factor, thereby yielding highersensitivities at the cost of reduced bandwidth and robustness. Operationwith the resonant frequencies separated by some prescribed amount, ormode-mismatched, is more common, particularly for automotiveapplications where robustness is critical.

Previously, gyroscope designs have been introduced aimed at robustoperation using an expanded sense-mode design space through increaseddegrees of freedom. Specifically, these devices use two coupled sensemasses forming a 2-DOF dynamic system with two sense mode resonantfrequencies and a wide region of constant amplitude between them. Whilethe gain and the bandwidth of this operational region is controlledsolely by the resonant frequency spacing, a constraint limited theminimal achievable spacing as the operational frequency of device withfixed size was increased. This is a direct effect of the dynamicvibration absorber type 2-DOF design, which utilized only twosuspensions thereby eliminating the ability to independently define thefrequency spacing and the coupling between the masses. In contrast, acomplete 2-DOF system consisting of two masses and three suspensionsalleviates this issue allowing for the arbitrary specification offrequency spacing independent of operational frequency.

What is needed is a interchangeable micromachined gyroscope that allowsfor both increased sensitivity and wide-bandwidth, robust operation in asingle device.

BRIEF SUMMARY OF THE INVENTION

The current invention is for a micromachined z-axis vibratory rategyroscope comprising a first 1-DOF drive subsystem restrained tooscillate substantially only in the drive mode, and having a drivesuspension and drive mass, and a 2-DOF sense subsystem restrained tooscillate substantially only in the sense mode, having at least twosense mode resonant frequencies and resiliently coupled to the 1-DOFdrive subsystem, where the 1-DOF drive subsystem has a drive frequencyindependent of the at least two sense mode resonant frequencies, whichdrive frequency is determined by selection of dynamic parameters for thedrive suspension and drive mass.

The 1-DOF drive subsystem of the gyroscope comprises an outer anchor, aninner anchor, an outer decoupling frame coupled to the outer anchor byat least one drive suspension that constrains the motion of the outerdecoupling frame to substantially only the drive axis, and an innerdecoupling frame coupled to the inner anchor and at least one drivesuspension that constrains the motion of the outer decoupling frame tosubstantially only the drive axis.

The 2-DOF sense subsystem of the gyroscope comprises a first sense massm_(a) coupled to one of either the inner decoupling frame or the outerdecoupling frame and restrained to oscillate substantially only in thesense direction, a second sense mass m_(b) coupled to the other one ofeither inner decoupling frame or outer decoupling frame and restrainedto oscillate substantially only in the sense direction, and anindependent flexure coupling the first and second sense masses to allowrelative motion of the first and second sense masses m_(a) and m_(b) insubstantially only in the sense direction.

The 1-DOF drive subsystem of the gyroscope further comprises a driveelectrostatic comb electrode for the actuation, detection and control ofthe inner and outer decoupling frames in the drive axis.

The 2-DOF sense subsystem of the gyroscope further comprises a senseelectrostatic electrode for the actuation, detection of the motioninduced by the Coriolis acceleration and for tuning of the resonantfrequencies of the first and second sense masses. The 2-DOF sensesubsystem further comprises means for independently controlling bothpeak spacing of the at least two sense mode resonant frequencies and theamount of coupling between the first and second sense masses.

The dynamic parameters of the 2-DOF sense subsystem of the gyroscope areselected so that the at least two resonant sense frequencies having apeak spacing with a predetermined bandwidth and where the dynamicparameters of the 1-DOF drive subsystem are selected to establish thedrive frequency within the predetermined bandwidth for a robustoperational mode.

Alternatively, the dynamic parameters of the 2-DOF sense subsystem ofthe gyroscope are selected so that the at least two resonant sensefrequencies are predetermined and where the dynamic parameters of the1-DOF drive subsystem are selected to establish the drive frequency onor near at least one of the at least two resonant sense frequencies fora precision operational mode.

The gyroscope further comprises first means for adjusting the dynamicparameters of the 1-DOF subsystem and/or 2-DOF subsystem duringoperation for a precision operational mode where the dynamic parametersof the 2-DOF sense subsystem are selected so that the at least tworesonant sense frequencies are predetermined and where the dynamicparameters of the 1-DOF drive subsystem are selected to establish thedrive frequency on or near at least one of the at least two resonantsense frequencies, or for adjusting the dynamic parameters of the 1-DOFsubsystem and/or 2-DOF subsystem during operation for a robustoperational mode where the dynamic parameters of the 2-DOF sensesubsystem are selected so that the at least two resonant sensefrequencies having a peak spacing with a predetermined bandwidth andwhere the dynamic parameters of the 1-DOF drive subsystem are selectedto establish the drive frequency within the predetermined bandwidth.

The current invention also provides for a method of operating aninterchangeable micromachined z-axis vibratory rate gyroscope capable ofoperating in a robust mode where a drive frequency of a 1-DOF drivesubsystem of the gyroscope is established in a peak spacing between atleast two resonant sense frequencies with a predetermined bandwidth of a2-DOF sense subsystem of the gyroscope and capable of operating in aprecision mode where a drive frequency of the 1-DOF drive subsystem ofthe gyroscope is established on or near one of the at least two resonantsense frequencies of the 2-DOF sense subsystem of the gyroscope,comprising interchangeably switching between the robust mode and theprecision mode during real-time operation.

The method of operating an interchangeable micromachined z-axisvibratory rate gyroscope further comprises operating in the robust modewhere the drive frequency of the 1-DOF drive subsystem of the gyroscopeis established in the peak spacing between at least two resonant sensefrequencies with the predetermined bandwidth of the 2-DOF sensesubsystem of the gyroscope after switching from the precision mode.

In an alternative embodiment, the method of operating an interchangeablemicromachined z-axis vibratory rate gyroscope further comprisesoperating in the precision mode where the drive frequency of the 1-DOFdrive subsystem of the gyroscope is established on or near one of the atleast two resonant sense frequencies of the 2-DOF sense subsystem of thegyroscope after switching from the robust mode.

In an alternative embodiment, the method step of where interchangeablyswitching between the robust mode and the precision mode duringreal-time operation comprises operating the gyroscope in the robust modeby selection of the dynamic parameters of the 1-DOF drive subsystem andthe 2-DOF sense subsystem and electrostatically tuning the 2-DOF sensesubsystem to switch to the precision mode of operation.

In yet another embodiment, the method step of where interchangeablyswitching between the robust mode and the precision mode duringreal-time operation comprises operating the gyroscope in the precisionmode by selection of the dynamic parameters of the 1-DOF drive subsystemand the 2-DOF sense subsystem and electrostatically tuning the 2-DOFsense subsystem to switch to the robust mode of operation.Alternatively, this method step may further comprise detecting theswitching between the robust mode and the precision mode duringreal-time operation by measuring quadrature magnitude output of thegyroscope.

In an alternative embodiment of the gyroscope, the gyroscope outlineabove further comprises a second 1-DOF drive subsystem restrained tooscillate substantially only in the drive mode, and having acorresponding drive suspension and corresponding drive mass and a second2-DOF sense subsystem restrained to oscillate substantially only in thesense mode, having at least two corresponding sense mode resonantfrequencies, and resiliently coupled to the second 1-DOF drivesubsystem, where the second 1-DOF drive subsystem has a drive frequencyindependent of the at least two corresponding sense mode resonantfrequencies, which corresponding drive frequency is determined byselection of dynamic parameters for the corresponding drive suspensionand corresponding drive mass, and wherein the first and second 1-DOFsubsystems are resiliently coupled to each in the drive mode, butcapable of being driven in antiphase to reject common mode inputs.

The 1-DOF drive subsystem of the alternative embodiment of the gyroscopefurther comprises a corresponding drive electrostatic comb electrode forthe actuation, detection and control of the inner and outer decouplingframes in the drive axis. Additionally, the second 2-DOF sense subsystemof the gyroscope further comprises a corresponding sense electrostaticelectrode for the actuation, detection of the motion induced by theCoriolis acceleration and for tuning of the resonant frequencies of thefirst and second sense masses.

The alternative embodiment of the gyroscope outlined above furthercomprises corresponding second means for adjusting the dynamicparameters of the second 1-DOF subsystem and/or second 2-DOF subsystemduring operation for a precision operational mode where the dynamicparameters of the second 2-DOF sense subsystem are selected so that theat least two corresponding resonant sense frequencies are predeterminedand where the corresponding dynamic parameters of the second 1-DOF drivesubsystem are selected to establish the corresponding drive frequency onor near at least one of the at least two corresponding resonant sensefrequencies, or for adjusting the dynamic parameters of the second 1-DOFsubsystem and/or second 2-DOF subsystem during operation for a robustoperational mode where the corresponding dynamic parameters of thesecond 2-DOF sense subsystem are selected so that the at least twocorresponding resonant sense frequencies having a peak spacing with apredetermined bandwidth and where the dynamic parameters of the second1-DOF drive subsystem are selected to establish the corresponding drivefrequency within the predetermined corresponding bandwidth.

The current invention also provides for yet another micromachined z-axisvibratory rate gyroscope formed on a substrate comprising an outerdecoupling drive frame restrained to oscillate substantially only in adrive direction, an inner decoupling drive frame restrained to oscillatesubstantially only in a drive direction, the inner and outer decouplingdrive frames being decoupled from each other and both resilientlycoupled to the substrate, a first sense mass m_(a) resiliently coupledto the outer decoupling drive frame and restrained to oscillatesubstantially only in the sense direction, a second sense mass m_(b)resiliently coupled to the outer decoupling drive frame and restrainedto oscillate substantially only in the sense direction, the first andsecond sense masses m_(a) and m_(b) being independently resilientlycoupled to each other to allow relative oscillation in the sensedirection, a drive electrostatic comb coupled to either the innerdecoupling drive frame or the outer decoupling frame or both, and asense electrostatic comb coupled to either the first sense mass or thesecond sense mass or both.

The current device is a new, z-axis micromachined gyroscope design thattakes advantage of a complete 2-DOF coupled sense mode allowingindependent adjustment of both the sense mode resonant frequencies andthe amount of coupling between the masses. This is accomplished throughthe introduction of a second, inner decoupling frame connected to acentral anchor.

Furthermore, the complete 2-DOF sense mode enables a new operationalmethod where the drive mode can be interchangeably placed between thesense mode peaks (called a “robust mode”) or mode-matched to one of the2-DOF sense mode resonant frequencies (called a “precision mode”). Thecontrol of bandwidth and coupling allows each mode to be independentlytailored in order to meet application specific objectives and allows theuser adjust the output to the changing environmental conditions.

The disclosed device takes advantage of a complete, fully coupled 2-DOFsense system which, unlike the previous implementations found in theprior art, allows for the specification of the sense mode resonantfrequencies and amount of coupling independent of operational frequency.This also eliminates the undesirable scaling effects that were a directeffect of the previous 2-DOF sense mode design. Additionally, the newdesign concepts presented herein enable a new operational paradigmallowing for interchangeable operation in both robust and precisionmodes.

The robust mode corresponds to operation between the 2-DOF sense moderesonant frequencies; this provides a response gain and bandwidthcontrolled by mechanical design through adjustment of the sense modefrequency spacing.

The precision mode of operation relies on mode-matching to one of thesense mode resonant frequencies. This resonant peak can be designed toprovide a gain advantage over comparable 1-DOF systems throughadjustment of the coupling between the masses. The precision mode gaincan also be increased independent of the robust mode gain and bandwidthby decreasing the atmospheric pressure in which the device operates.Also, matching to the second, anti-phase resonance eliminates substrateenergy dissipation enabling higher achievable sensitivities. Thus, thepresented concepts are essential to enabling the interchangeableoperation due to the independent tuning of the robust and precision modegains.

While the apparatus and method has or will be described for the sake ofgrammatical fluidity with functional explanations, it is to be expresslyunderstood that the claims, unless expressly formulated under 35 USC112, are not to be construed as necessarily limited in any way by theconstruction of “means” or “steps” limitations, but are to be accordedthe full scope of the meaning and equivalents of the definition providedby the claims under the judicial doctrine of equivalents, and in thecase where the claims are expressly formulated under 35 USC 112 are tobe accorded full statutory equivalents under 35 USC 112. The inventioncan be better visualized by turning now to the following drawingswherein like elements are referenced by like numerals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a schematic diagram of the dynamic elements of the gyroscope.

FIG. 1b is a schematic diagram of the structural organization of themicormachined gyroscope with 2-DOF sense modes allowing interchangeablerobust and precision operation in real-time.

FIG. 2a is a top plan view of an embodiment of the gyroscope with asmaller second sense mass and a larger inner decoupled frame comprisinga plurality of capacitive electrodes.

FIG. 2b is a top plan view of an embodiment of the gyroscope withcapacitive electrodes both first and second sense masses and a smallerinner decoupling frame with no electrodes.

FIG. 3 is a graphical representation of frequency response magnitude indB versus frequency in Hz for the drive and sense mode frequencyresponses highlighting the robust and precision operational regions.

FIG. 4 is a graphical representation of damping ratio versus percentagecoupling for the gain ratio of the 2-DOF sense mode of the currentgyroscope versus 1-DOF sense modes found in the prior art.

FIG. 5 is an example layout of the micromachined gyroscope.

FIG. 6 is a graphical representation of frequency response magnitudeversus frequency for sense mode frequency responses in air or ambientconditions and vacuum of the micomachined gyroscope, both for lowcoupling stiffness and high coupling stiffness as shown in the figure onthe left and right graphs respectively.

FIG. 7 is a schematic diagram of the actuation and detectionarchitecture used for the micromachined gyroscope.

FIG. 8 is a graphical representation of quadrature magnitude in dBversus the tuning voltage for the micromachined gyroscope with an insetof frequency versus tuning voltage for the micromachined gyroscope.

FIG. 9 is a graphical representation of voltage versus angular rate forboth robust and precision modes of the micromachined gyroscope in air.

FIG. 10 is a top plan view of the tuning fork embodiment of themicromachined gyroscope.

The invention and its various embodiments can now be better understoodby turning to the following detailed description of the preferredembodiments which are presented as illustrated examples of the inventiondefined in the claims. It is expressly understood that the invention asdefined by the claims may be broader than the illustrated embodimentsdescribed below.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Lumped structural models of the new gyroscope concept is presented inFIGS. 1a and 1b . The gyroscope is generally denoted by referencenumeral 10. The device, like previous multi-DOF sense mode devices, iscomprised of a conventional drive mode that is mechanically decoupledfrom a 2-DOF sense mode dynamic system formed by the two coupled sensemasses, m_(a) 14 and m_(b) 12. Sense mass m_(b) 12 is also known as adetection mass. The gyroscope 10 further comprises a third suspensionelement, namely an outer decoupling frame 20 in the sense mode forming acomplete 2-DOF coupled system. The outer decoupling frame 20 is coupledto an outer anchor 24 by a plurality of drive suspension elements 38.The suspension elements 38 constrain the motion of the outer decouplingframe 20 to only along the drive axis. Due to the frame decoupling andsymmetry of the design, the addition of the outer decoupling frame 20 ismade possible through the use of a second, inner decoupling frame 16suspended in the drive mode relative to a central or inner anchor 18seen in FIG. 1 b. The inner decoupling frame 16 is similarly coupled toan inner anchor 18 by drive suspension elements 28 which constrain themotion of the inner decoupling frame 16 to only the drive axis.

Damping between outer frame 20 and the substrate or outer anchor andinner frame 16 and the substrate or inner anchor 18 is represented bydampers c_(d), 39 in FIG. 1a . Damping between sense mass m_(a) 14 andouter frame 20 is represented by damper c_(a), 41 in FIG. 1a . Dampingbetween sense mass m_(b) 12 and inner frame 16 is represented by damperc_(b), 45 in FIG. 1 a.

The first sense mass m_(a) 14 is coupled to the outer decoupling frame20 by sense suspension elements 40. Similarly, the second sense massm_(b) 12 is coupled to the inner decoupling frame 16 by sense suspensionelements 43. The first and second sense masses m_(a) 14 and m_(b) 12 arecoupled together by a coupling flexure 42 that can be adjustedindependently of the other drive and sense suspensions 28, 38, 40, 43.

Several different physical layout implementations of the proposedgyroscope concept have been considered and implemented; two of theseembodiments are presented in FIGS. 2a and 2b . The first embodiment,FIG. 2a , contains a plurality of lateral comb electrodes 29 on both theinner and outer decoupling frames 16, 20, as well as a plurality ofparallel plate electrodes 26 on the outer frame 20 for resonantfrequency tuning. Also, the smaller sense mass m_(b) 12 is placed on theoutside of sense mass m_(a) 14 to increase the number of parallel plateelectrodes 26 and for easier wire-bonding access to the gyroscope 10.

The second embodiment, FIG. 2b , is implemented with a much smallerinner frame 16 with no electrodes. A plurality of lateral combelectrodes 29 and a plurality of parallel plate electrodes 26 aredisposed on the outer sense mass m_(a) 14 for actuation, detection, andtuning. Additionally, a plurality of parallel plate electrodes 26 aredisposed on the inner sense mass m_(b) 12 for detection, control, andtuning.

The complete 2-DOF sense mode provides the device with distinctadvantages over the previous multi-DOF designs found in the prior artwhich relied on only two suspensions. This includes the ability toachieve the desired sense mode resonant frequencies independent ofoperational frequency as well as control over the degree of couplingbetween the sense masses.

Since the drive mode of the illustrated disclosure is a conventional1-DOF resonant system, the desired operational frequency, ω_(d), can beobtained independently of the sense mode through adjustment of the drivesuspension or mass of the inner and outer decoupling frames 16, 20. Onthe other hand, the sense mode is a coupled 2-DOF system where thenatural frequencies are determined by eigenvalues in terms of thestructural frequencies ω² _(a)=(k_(a)+k_(c))/m_(a) and ω²_(b)=(k_(b)+k_(c))/m_(b), as well as the coupling between the masses, ω²_(c)=k_(c)/√(m_(a)m_(b)). If the desired sense mode resonantfrequencies, ω_(1,2) are specified in terms of the drive frequency,ω_(d), and the sense mode frequency spacing, Δ=ω₂−ω₁, the followingsense mode design equations can be found,

$\begin{matrix}{{\omega_{a,b}^{2} = {\omega_{d}^{2} + {\left( \frac{\Delta}{2} \right)^{2} \pm \sqrt{{\omega_{d}^{2}\Delta^{2}} - \omega_{c}^{4}}}}},} & (1)\end{matrix}$which assumes the drive is equally spaced from the sense moderesonances.

As shown in equation (1), the amount of coupling between the masses, ω²_(c), can be adjusted independently from the structural frequencies.There is, however, a limited range of values that can be specified whilemaintaining a physically meaningful system. Therefore, the coupling canbe expressed in terms of a percentage of the maximum couplingω_(c) ²=εω_(d)Δ, where 0<ε<1,  (2)where ε is the coupling parameter. Substituting (2) into (1) gives,

$\begin{matrix}{{\omega_{a,b}^{2} = {\omega_{d}^{2} + {\left( \frac{\Delta}{2} \right)^{2} \pm {\omega_{d}\Delta\sqrt{1 - \varepsilon^{2}}}}}},} & (3)\end{matrix}$which are the sense mode design equations in terms of desiredoperational frequency, ω_(d), sense mode resonant frequency spacing, Δ,and the amount of coupling, ε. From equations (3) and (2), thestiffnesses required to achieve the desired parameters can be found,k _(a)=ω_(a) ² m _(a) =k _(c),k _(b)=ω_(b) ² m _(b) −k _(c),k _(c) =nω _(d)Δ√{square root over (m _(a) m _(b))},assuming the value of the sense masses 12, 14 are known.

While the current device removes a constraint with earlierimplementations, it also introduces a new operational method formulti-DOF gyroscopes: interchangeable operation in both precision androbust modes in a single device. This is enabled by the ability toindependently control both the peak spacing and the amount of couplingbetween the masses 12, 14 of the 2-DOF sense mode. The concept of bothrobust and precision modes of operation is shown in FIG. 3 whichpresents the conceptual frequency responses of both the drive andcomplete 2-DOF sense mode.

Similar to previous multi-DOF devices, the robust mode corresponds tooperation in the region between the sense mode resonant frequencies;precision operation, however, the current device consists ofmode-matching the drive to the sense mode anti-phase resonant frequency.As with conventional devices, the resonant amplitude of the precisionmode can be increased with decreasing pressures resulting in highersensitivities whereas the amplitude of the robust region is mainly afunction of resonant frequency spacing. Thus, the gains of each mode canbe controlled independently enabling both high sensitivity and robust,wide-bandwidth operation at reduced pressures.

The current device also comprises means for obtaining increased resonantgain over an equivalent 1-DOF sense mode for similar damping conditions.This is illustrated with the simulation results graphed in FIG. 4showing the gain ratio in dB between the 2-DOF anti-phase resonantfrequency and a 1-DOF system of m_(a)+m_(b) at the same frequency. Inthe simulation, the total damping c_(a)+c_(b), corresponding to a 1-DOFquality factor of 500, remained fixed for changes in the couplingparameter, ε, and the 2-DOF damping ratio, c_(b)/c_(a). There existsdesign parameters where the 2-DOF system has over a 6 dB gain advantageversus the equivalent 1-DOF system. This advantage is achieved withlarger damping on the detection mass m_(b) 12 meaning that increasingthe sensing capacitance, i.e. more sensing plates or smaller capacitivegaps, is advantageous for the presented design.

Prototypes of the disclosed gyroscope device used for the experimentalcharacterization presented below were fabricated using an in-house, twomask, wafer scale SOI process with a conductive 50 μm device layer and a5 μm buried oxide. First, a front side metallization process definedbonding pads via lift-off followed by a Deep Reactive Ion Etching (DRIE)step using a Surface Technology Systems (STS) Advanced Silicon Etching(ASE) tool. The minimum feature size of the overall process used todefine the capacitive gaps was 5 μm. The perforated structures werereleased using a timed HF etch followed by dicing, packaging, and wirebonding. A scanning electron micrograph of a fabricated prototype of thecurrent device is presented in FIG. 5.

Several different sense mode systems were designed with varying degreesof coupling between the masses 12, 14 in order to determine its affecton the response gain. Experimental frequency responses in air and vacuumare presented in FIG. 6 for both high and low stiffness sense modes. Foratmospheric operation, the low stiffness system is optimal as itprovides a higher precision mode gain improvement (10 dB versus 7 dB)versus the 240 Hz wide robust gain regions. At vacuum, however, the highcoupling stiffness system provides the larger precision mode improvementof more than 40 dB versus the robust region. Thus, for vacuum operation,a high stiffness system is desirable as it provides over 50 timesimprovement in precision mode operation versus atmospheric pressure.

The disclosed device was characterized using constant angular rates inorder to verify its functionality as a gyroscope 10 for both robust andprecision modes. The paragraphs below detail the biasing used for theexperiment, a method of mode tuning for switching between theoperational modes, and finally the scale factor calibration for bothmodes.

FIG. 7 presents a schematic of the actuation and detection scheme usedfor the angular rate experimental characterization. For drive modeactuation, an AC driving voltage, V_(d)(t), plus a DC potential, V_(d),was applied to the fixed electrodes 29 a while a high frequency ACcarrier voltage, v_(c)(t), plus a sense mode tuning voltage, V_(t), wasapplied to the mobile mass of outer frame 20, through electrodes 29 b.In order to maintain a constant driving amplitude, the total DC drivingpotential, V_(d)−V_(t), was fixed during sense mode tuning experiments.

In the sense mode, a differential detection scheme using cascadedtrans-impedance and instrumentation amplifiers was used to pick up themotion of mass m_(b) 12. The differential signal was then demodulated atthe carrier frequency followed by a second demodulation at the drivefrequency using AMETEK Model 7265 Lock-In Amplifiers 30 as seen in FIG.7. The use of the high frequency carrier allows for the separation ofuseful sense signals from parasitics.

The fabricated gyroscope 10 used in the experimental characterizationwas designed with the drive mode natural frequency of 5.44 kHz betweenthe lower and higher sense mode resonances of 5.2 and 5.6 kHzrespectively. Therefore, mode-matching for the presented gyroscope 10required electrostatic tuning of the sense mode in order to match thehigher anti-phase resonant frequency to the drive. By adjusting the DCtuning voltage, V_(t), applied to the mass m_(b) the sense mode resonantfrequencies can be shifted down in frequency as shown in the inset ofFIG. 8. As the voltage is increased, both sense mode frequencies areshifted until the higher sense mode frequency crosses the drive mode atapproximately 35 V.

While the sense mode tuning curves indicate that the gyroscope 10 can bemode-matched, it does not ensure precise matching is achieved. The zerorate output of the gyroscope 10, or quadrature, however, can bemonitored for changes in tuning voltage with a maximum output occurringwhen the drive and sense frequencies are equal. The quadrature output indB versus tuning voltage is presented in FIG. 8 for the gyroscope 10operated in 5.6 Torr vacuum. For low tuning voltages (0-15 V), the drivemode is in the robust operational mode between the peaks; as the voltageis increased, the quadrature signal increases until a maximum isachieved at 34 V indicating a mode-matched condition, while even highervoltages result in decreasing signals.

To verify both operational modes, the gyroscope 10 was characterized inatmospheric pressure at constant angular rates using an Ideal Aerosmith1291 BR rate table. The actuation and detection scheme shown in FIG. 7was used with tuning voltages of 0 V and 34 V for robust and precisionmodes, respectively, while the total driving voltage was kept constant.For robust operation, the experimentally obtained scale factor was 0.282mV/deg/s while for precision operation it was 0.690 mV/deg/s. Thus,precision mode resulted in a 2.4 times improvement at atmosphericpressure; for operation at 1 mTorr vacuum, however, a possibleimprovement of 50 times can be obtained for precision mode while leavingthe robust region unaffected as shown in FIG. 6.

In an alternative embodiment, the gyroscope 10 may be used as a tuningfork 32 that maintains the interchangeable operation in robust andprecision modes presented above. A physical implementation of the tuningfork embodiment is shown in FIG. 10 which consists of two, fully coupledsense modes for the right tine 34 and left tine 36. Each tine 34, 36comprises a sense mass m_(a) 14, a sense mass m_(b) 12, an inner anchor22, an inner frame 16, an outer frame 20, and a plurality of parallelplate electrodes 26 and lateral comb electrodes 29. The tuning fork 32differs from the previously described gyroscope 10 through the presenceof two tines 34, 36 coupled in the drive mode forming a 2-DOF system. Bydriving the left and right tines 34, 36 into anti-phase motion, theoverall tuning fork device 32 can reject common mode inputs, such asaccelerations, while also allowing better momentum balance for increasedquality factor operation.

Many alterations and modifications may be made by those having ordinaryskill in the art without departing from the spirit and scope of theinvention. Therefore, it must be understood that the illustratedembodiment has been set forth only for the purposes of example and thatit should not be taken as limiting the invention as defined by thefollowing invention and its various embodiments.

Therefore, it must be understood that the illustrated embodiment hasbeen set forth only for the purposes of example and that it should notbe taken as limiting the invention as defined by the following claims.For example, notwithstanding the fact that the elements of a claim areset forth below in a certain combination, it must be expresslyunderstood that the invention includes other combinations of fewer, moreor different elements, which are disclosed in above even when notinitially claimed in such combinations. A teaching that two elements arecombined in a claimed combination is further to be understood as alsoallowing for a claimed combination in which the two elements are notcombined with each other, but may be used alone or combined in othercombinations. The excision of any disclosed element of the invention isexplicitly contemplated as within the scope of the invention.

The words used in this specification to describe the invention and itsvarious embodiments are to be understood not only in the sense of theircommonly defined meanings, but to include by special definition in thisspecification structure, material or acts beyond the scope of thecommonly defined meanings. Thus if an element can be understood in thecontext of this specification as including more than one meaning, thenits use in a claim must be understood as being generic to all possiblemeanings supported by the specification and by the word itself.

The definitions of the words or elements of the following claims are,therefore, defined in this specification to include not only thecombination of elements which are literally set forth, but allequivalent structure, material or acts for performing substantially thesame function in substantially the same way to obtain substantially thesame result. In this sense it is therefore contemplated that anequivalent substitution of two or more elements may be made for any oneof the elements in the claims below or that a single element may besubstituted for two or more elements in a claim. Although elements maybe described above as acting in certain combinations and even initiallyclaimed as such, it is to be expressly understood that one or moreelements from a claimed combination can in some cases be excised fromthe combination and that the claimed combination may be directed to asubcombination or variation of a subcombination.

Insubstantial changes from the claimed subject matter as viewed by aperson with ordinary skill in the art, now known or later devised, areexpressly contemplated as being equivalently within the scope of theclaims. Therefore, obvious substitutions now or later known to one withordinary skill in the art are defined to be within the scope of thedefined elements.

The claims are thus to be understood to include what is specificallyillustrated and described above, what is conceptionally equivalent, whatcan be obviously substituted and also what essentially incorporates theessential idea of the invention.

We claim:
 1. A micromachined z-axis vibratory rate gyroscope formed on asubstrate comprising: an outer anchor; an inner anchor; an outerdecoupling drive frame coupled to the outer anchor restrained tooscillate substantially only in a drive direction; an inner decouplingdrive frame coupled to the inner anchor restrained to oscillatesubstantially only in a drive direction, the inner and outer decouplingdrive frames being decoupled from each other and both resilientlycoupled to the substrate; a first sense mass m_(a) resiliently coupledto the outer decoupling drive frame and restrained to oscillatesubstantially only in the sense direction; a second sense mass m_(b)resiliently coupled to the inner decoupling drive frame and restrainedto oscillate substantially only in the sense direction, the first andsecond sense masses m_(a) and m_(b) being independently resilientlycoupled to each other to allow relative oscillation in the sensedirection; a drive electrostatic comb coupled to either the innerdecoupling drive frame or the outer decoupling frame or both; and asense electrostatic comb coupled to either the first sense mass or thesecond sense mass or both.